#include <bits/stdc++.h>
using namespace std;
void setmin(int& a, int b) {
if (a > b) {
a = b;
}
}
const int INF = 1e9;
const int MAXN = 405;
int N;
char S[MAXN];
vector<int> locs[3];
int g[3][MAXN][3];
int dp[2][MAXN][MAXN][3];
map<char, int> mp = {
{'R', 0},
{'G', 1},
{'Y', 2}
};
int get_cost(int a, int b, int c, int x, int p) {
return max(0, a - g[x][p][0]) +
max(0, b - g[x][p][1]) +
max(0, c - g[x][p][2]);
}
int main() {
scanf("%d", &N);
scanf("%s", S);
for (int i = 0; i < N; i++) {
locs[mp[S[i]]].push_back(i);
}
for (int a = 0; a < 3; a++) {
int tmp[3] = {};
for (int i = 0; i < int(locs[a].size()); i++) {
for (int b = 0; b < 3; b++) {
while (tmp[b] < int(locs[b].size()) && locs[b][tmp[b]] < locs[a][i]) {
tmp[b]++;
}
g[a][i][b] = tmp[b];
}
}
}
int cur = 0, nxt = 1;
dp[cur][0][0][0] = 0;
dp[cur][0][0][1] = dp[cur][0][0][2] = INF;
for (int n = 0; n < N; n++, swap(cur, nxt)) {
for (int a = 0; a <= n + 1; a++) {
for (int b = 0; a + b <= n + 1; b++) {
fill_n(dp[nxt][a][b], 3, INF);
}
}
for (int a = 0; a <= n; a++) {
for (int b = 0; a + b <= n; b++) {
int c = n - a - b;
for (int x = 0; x < 3; x++) {
if (dp[cur][a][b][x] == INF) continue;
if ((a + b == 0 || x != 0) && a < int(locs[0].size())) {
setmin(dp[nxt][a + 1][b][0], dp[cur][a][b][x] + get_cost(a, b, c, 0, a));
}
if (x != 1 && b < int(locs[1].size())) {
setmin(dp[nxt][a][b + 1][1], dp[cur][a][b][x] + get_cost(a, b, c, 1, b));
}
if (x != 2 && c < int(locs[2].size())) {
setmin(dp[nxt][a][b][2], dp[cur][a][b][x] + get_cost(a, b, c, 2, c));
}
}
}
}
}
int ans = INF;
for (int c = 0; c < 3; c++) {
setmin(ans, dp[cur][locs[0].size()][locs[1].size()][c]);
}
if (ans == INF) ans = -1;
printf("%d\n", ans);
return 0;
}
